The present invention relates to a process for preparing 3-[(4S)-8-bromo-1-methyl-6-(2-pyridinyl)-4H-imidazo[1,2-a][1,4]benzodiazepine-4-yl]-propionic acid methyl ester and the benzene sulfonate salt thereof, starting from 3-[(S)-7-Bromo-2-((R and/or S)-2-hydroxy-propylamino)-5-pyridin-2-yl-3H-benzo[e][1,4]diazepin-3-yl]-propionic acid methyl ester or 3-[(S)-7-Bromo-2-((R)-2-hydroxy-propylamino)-5-pyridin-2-yl-3H-benzo[e][1,4]diazepin-3-yl]-propionic acid methyl ester, and new compounds useful as starting material or intermediate in that process.
WO 00/69836 describes short-acting benzodiazepines that include a carboxylic ester moiety and are inactivated by non-specific tissue esterases. An organ-independent elimination mechanism is predicted to be characteristic of these benzodiazepines, providing a more predictable and reproducible pharmacodynamic profile. The compounds are suitable for therapeutic purposes, including sedative-hypnotic, anxiolytic, muscle relaxant and anticonvulsant purposes. The compounds are short-acting CNS depressants that are useful to be administered intravenously in the following clinical settings: preoperative sedation, anxiolysis, and amnestic use for perioperative events; conscious sedation during short diagnostic, operative or endoscopic procedures; as a component for the induction and maintenance of general anesthesia, prior and/or concomitant to the administration of other anaesthetic or analgesic agents; ICU sedation.
One of the compounds disclosed in that document is 3-[(4S)-8-bromo-1-methyl-6-(2-pyridinyl)-4H-imidazo[1,2-a][1,4]benzodiazepine-4-yl]-propionic acid methyl ester of formula (F) below

WO 00/69836 teaches a process for preparing the above compound of formula (F), which comprises:    (a) preparing 3-[(S)-7-bromo-2-oxo-5-pyridin-2-yl-2,3-dihydro-1H-1,4-benzodiazepin-3-yl]-propionic acid methyl ester of formula (D)
by reacting (2-amino-5-bromo-phenyl)-pyridin-2-yl-methanone of formula (A)
in chloroform with an alpha-Fmoc-protected-amino acid chloride (obtained by reacting FMOC-Glu(OMe)-OH and oxalylchloride in dichloromethane), treating the obtained amide with triethylamine in dichloromethane, then with acetic acid in 1,2-dichloroethane, isolating the compound of formula (D), and    (b) reacting the compound of formula (D) with a suspension of sodium hydride in THF, treating the reaction mixture with bis-morpholinophosphochloridate (BPMC) in THF, filtering the reaction mixture, reacting the filtrate with DL-1-amino-propanol, purifying the alcoholic adduct obtained, treating that purified alcoholic adduct with a mixture of DMSO and oxalyl chloride in dichloromethane, treating the reaction mixture with triethylamine, diluting with ethyl acetate, washing with aqueous solutions and concentrating to give a foam, treating that foam with a catalytic amount of p-toluenesulfonic acid, neutralizing the solution with sodium hydrogenocarbonate and isolating the compound of formula (F).
WO 2008/007071 discloses a method of preparing the besylate salt (P) of the above compound of formula (F) by adding benzene sulfonic acid to a solution of that compound in toluene or ethyl acetate, stirring, filtering, washing with toluene or ethyl acetate and drying under vacuum. That method yields 3-[(4S)-8-Bromo-1-methyl-6-(2-pyridinyl)-4H-imidazo[1,2-a][1,4]benzodiazepine-4-yl]propionic acid methyl ester benzene sulfonate (P) which is taught to be a particularly interesting active pharmaceutical ingredient (API).
The process for preparing the direct precursor of that API, namely 3-[(4S)-8-bromo-1-methyl-6-(2-pyridinyl)-4H-imidazo[1,2-a][1,4]benzodiazepine-4-yl]propionic acid methyl ester of formula (F), or that API, 3-[(4S)-8-Bromo-1-methyl-6-(2-pyridinyl)-4H-imidazo[1,2-a][1,4]benzodiazepine-4-yl]propionic acid methyl ester benzene sulfonate (P), starting from the compound of formula (A) disclosed in WO 00/69836 is not satisfying for an industrial preparation, notably because of the high number of steps, the low overall yield and the insufficient optical purity of the compounds obtained at the different steps.
The objective of the invention is to find a process for preparing compound (P) and precursors thereof that does not have the above drawbacks.
That objective is attained by the invention as defined in the appended claims.